Unmanned Aerial Vehicles (UAVs) are used in a wide range of applications, both civilian and military, including inspection, intelligence, reconnaissance and rescue missions. UAV designs ranges from large fixed wing jets to smaller rotary wing aircraft with one or more rotors. Progress in the electronics industry over the past decades have made it possible to shrink the components necessary for an unmanned aerial vehicle system to become palm sized, a so-called Micro Air Vehicle (MAV). These vehicles can for example lift a camera and transmit video images, while still being highly maneuverable.
Regardless of the size, rotary wing aircraft like helicopters, are typically sustained by one or more rotors, rotating about vertical rotor shafts. In conventional helicopters, the amount of thrust from the rotor and the direction of the thrust can be controlled by changing the pitch angle of the rotor blades. The horizontal direction of flight and the stability of the helicopter are controlled by cyclically adjusting the pitch angle of the rotor blades through a so-called swash plate. Cyclically adjusting the pitch angle means that the pitch angle of each rotor blade is adjusted from a maximum in one particular position to a minimum 180 degrees later in the rotation. When the blade pitch alters like this, the initially vertical thrust tilts, and thereby generates a horizontal vector, moving the helicopter in the desired direction. By collectively changing the blade pitch of all the rotor blades, i.e. change the blade pitch of all blades with the same amount, or by changing the rotational speed of the rotor, the helicopter can be controlled in a vertical direction.
The alteration in blade pitch angle of the rotor blades is typically done by operating control rods attached to the swash plate, a unit consisting of a rotating disc and a non-rotating disc rotationally connected to each other. Typically, the inner end of each blade is connected to the rotating disc via pitch links, while the control rods are attached to the non-rotating disc. Operating the control rods incline or decline the swash plate at one or several points. As the rotor rotates, the blade pitch angle of the rotor blades are cyclically adjusted through the rotation as they move over the section inclined or declined by the swash plate control rods.
The control rods in a rotary wing aircraft are typically attached to, or constitutes of, servos that operate the movement of the control rods on input signals from the pilot, making the aircraft move in the desired direction. However, in light of advances in MAV technology, and to enable simpler and lower-cost aircraft, it is desirable to utilize a design that does not rely on the previously mentioned swash plate and servos for maneuvering an aircraft.
JP 3-2298463 (U.S. Pat. No. 5,259,729), Fujihira et al. elaborates on a method where one can cyclically control the blade pitch angle of the rotor blades in a two-bladed rotor without the use of a swash plate and servos, thereby controlling the movement of the aircraft. A sensor measures the position of the rotor blades, while a connecting member is, at the top, attached to the rotor blade assembly and at, the bottom, fixed to a circular plate that rotates with the rotor shaft. The connecting member is made of flexible material and is adjusted to fit a certain rotor torque.
The connecting member transfers the rotor torque from the rotor shaft to the rotor. Due to air resistance and the rotational inertia of the rotor, any increase or decrease in the torque applied to the rotor will not immediately change the rotational speed of the rotor, but instead bend the connecting member. This bending or deformation, in turn, generates a corresponding tilting moment that tilts the rotor blades and alters their blade pitch angle. Since the two blades in the rotor blade assembly are connected, and can tilt about a common blade pitch axis, an alternation in torque will give an asymmetrical blade pitch angle, i.e. increase in one blade and decrease in the opposite blade, ultimately creating a dissymmetry in lift. The dissymmetry in lift or thrust will tilt the rotor and move the aircraft in a horizontal direction as long as the alternating torque is repeated at the same point in the rotational plane. As the torque is returned to the nominal level, the flexible characteristic of the connecting member implies that the rotor blades level out and the movement of the aircraft ends.
A disadvantage of the design shown in JP 3-2298463 is, however, that it relies on a nominal value of Revolutions per Minute (RPM) of the rotor and a nominal torque of the rotor to avoid extensive vibrations in the rotor. If an average torque applied to the rotor, from a motor of the aircraft, is above the nominal value, the rotor blade closest to the connecting member will always have a higher blade pitch angle than the opposite blade. If the average torque applied to the rotor is below the nominal value, the rotor blade closest to the connecting member will always have a lower blade pitch angle than the opposite blade. In both cases, the sustained difference in blade pitch angle between the two rotor blades will result in unacceptable vibrations and resonance in the aircraft. One example is under the appearance of light wind or turbulence, the aircraft will then immediately start to climb or descend. To counter this, the rotor RPM required to keep the aircraft vertically stable will result in a rotor torque that, most of the time, is different from the nominal torque. This again results in large vibrations, making it unsuited for any practical applications. In challenging conditions, or if high maneuverability is required, the performance of the design described in JP 3-2298463 is poor.
Operations outside the nominal torque will also result in the connecting member reaching a mechanical limit preventing further adjustments in blade pitch angle. This again results in a significant reduced level of blade pitch control. The reduction in control will be more evident the further away from the nominal torque the rotor is operated. In other words, the design described in JP 3-2298463 is best suited for indoor toy helicopters, but the design is not possible to use in an outdoor operational helicopter that requires high maneuverability and stable flight with a minimum of vibrations that could affect its pilot, autopilot and/or cameras carried therewith.
US 2011143628 describes an unconventional rotary wing aircraft suited for toy applications. The aircraft design is very different from a conventional helicopter, but it utilizes a cyclic rotor control system that is similar to the one described in JP 3-2298463. This aircraft is also controlled by cyclic changes in the torque applied to a two-bladed rotor. In this case, the rotor is injection molded in one piece using a flexible plastic material. The rotor control described in US 2011143628 operates, however, in the same way as the rotor control described in JP 3-2298463, and has the same limitations.
US 2004198136 A1 teaches a method and a system for controlling pitch of the rotor blades of a remote control ultralight helicopter, with at least one rotor blade. According to the invention, the adjustment of the pitch of the at least one rotor blade is achieved by means of a force, in particular a torsion force directly applied to the rotation axis of the rotor blade. The force is generated by a magnetic field, variable by the electrical control of at least one coil which is not part of an electric motor.
WO 2005087587 shows several rotor blade adjustment systems influencing aerodynamic force that does not require a swash plate, but reacts to cyclic and/or non-cyclic accelerations of a rotation variable supplied to the rotor. According to the desired control movement, the driving force is modulated and the control signal is transmitted as torque to the rotor by means of the rotor shaft to alter the position of the blades and thereby the movement of the aircraft.
There are even more examples of designs that aim to control the blade pitch angle of rotary wing aircraft by altering the torque to the rotor, e.g. Penn State University WO2014160526. Common to all of the examples is, however, the dissymmetry in blade pitch angle or orientation that occur if the rotor is operated at a RPM or torque level outside of the nominal design value, or if large cyclic control forces are required. In all the known designs, this dissymmetry will limit the possible blade pitch range and it will create undesired or unacceptable vibrations.
A thrust-generating rotor assembly for both small MAVs and larger rotary wing aircraft, without the disadvantages and limitations discussed above, would enable simpler, better and lower cost systems.